This application is a national stage of PCT/EP 99/04526 filed Jul. 1, 1999 and based, in turn, upon German national application 198 30 697.0 filed Jul. 8, 1998 under the International Convention.
This invention relates to a process for removing relatively coarse-grained solids from a stationary fluidized bed, which contains solids of different grain sizes, wherein fluidizing gas is introduced into the fluidized bed from the bottom, distributed through a jet bank.
Stationary fluidized beds of this kind are generally known. They are characterized by a more or less precisely defined bed surface, which moves for instance like a boiling liquid. During an extended operation of the fluidized bed solids of a relatively large grain size accumulate in the lower portion of the bed above the jet bank, as they are not fluidized or are hardly fluidized. This accumulation of coarse grain can be disturbing in many cases, as it can for instance lead to different dwell times of the solids in the fluidized bed and also to an overheating and sintering in the vicinity of the jet bank. The withdrawal of this coarse grain under the influence of gravity downwards through the jet bank or laterally through the container wall requires a considerable effort in terms of equipment and control technology.
It is the object of the invention to be able to at least partly and easily remove the relatively coarse-grained solids accumulating in the lower portion of a stationary fluidized bed.
In accordance with the invention this is achieved in the above-mentioned process in that by means of at least one upwardly directed gas jet supplied separate from the fluidizing gas part of the solids disposed above the jet bank are blown into a conveying tube, and these solids are pneumatically removed from the fluidized bed through the conveying tube. Since the relatively coarse-grained solids accumulate directly above the jet bank, the same are, as far as they are disposed in direct vicinity of the inlet opening of the conveying tube, blown out of the fluidized bed through the conveying tube together with the fine-grained solids present near the inlet opening. The solids discharged in this way car, be separated into a coarse grain fraction and a fine grain fraction by screening them outside the fluidized bed, where for instance the fine grain fraction is directly recirculated to the fluidized bed and the coarse grain fraction is first of all comminuted or subjected to a special treatment in a second container.
The solids in the fluidized bed may have any temperature, the spectrum of grain sizes may include grain sizes in the range from 0.1 to 10 mm, but there are no absolute size limits for this method.
The gas velocities of the fluidizing gas usually lie in the range from 0.1 to 1 m/sec, where these figures are measured as empty-tube velocities, here and later on. One or several gas jets are upwardly directed from the bottom against the inlet opening of the conveying tube, they entrain the solids and guide them through the conveying tube, through which they leave the fluidized bed. The gas velocities in the conveying tube usually are at least twice as high as the velocity of the fluidizing gas in the fluidized bed. Usually, the gas velocities in the conveying tube are twice to five times as high as in the fluidized bed.
It may be sufficient to install in the fluidized bed a single conveying tube with at least one associated gas jet coming from the bottom, but there may very well also be provided a plurality of conveying tubes at different points in the fluidized bed, to each of which at least one gas jet is associated.